1) Some monitors have multiple inputs (usually VGA and DVI connectors) and have a switch to change between them. This can either by a physical switch or alternatively you've got to select the right input through a menu on the monitor.
2) If the monitor is working, but not receiving a signal, the monitor will usually display something to that affect on the screen. Does it appear to be receiving a signal at all?
3) Check cables. If necessary try changing the cables for a set that's known to work (borrow from friends, neighbours .. or work). If using DVI to VGA convertors, try changing them for a different module.